MATEC Web of Conferences 21, 08007 (2015)
DOI: 10.1051/matecconf/20152108007
C Owned by the authors, published by EDP Sciences, 2015
Effect of higher heating rate during continuous annealing on
microstructure and mechanical properties of cold-rolled
590 MPa dual-phase steel
Yonggang Deng, Hongshuang Dia , Jiecen Zhang, and Liqing Chen
State Key Laboratory of Rolling and Automation, Northeastern University, 3-11 Wenhua Road,
Shenyang 110819, China
Abstract. In this presentation, the effect of higher heating rate in continuous annealing on
microstructure and mechanical properties of a cold-rolled 590 MPa ferrite-martensite dualphase steel were investigated by using microstructural observation and mechanical property
measurement. The results show that compared with the conventional continuous annealed
steels heated at a rate of 5 ◦ C/s (CA), the average ferrite grain sizes heated at a higher
rate (300 ◦ C/s, HRA) was obviously refined from 15.6 m to 5.3 m. The morphology of
martensite is observed to shift from network along ferrite grain boundaries to uniformly
dispersed in the final DP590 microstructure. Twinned substructure of martensite can be
found when heated at a higher heating rate in continuous annealing. EBSD orientation maps
show that the fraction of low angle grain boundary is increased in HRA sample compared
to CA sample. The HRA sample has excellent mechanical properties when compared to the
CA sample. The variations of strength, elongation, strain hardening behavior and fracture
mechanism of the this DP590 steel with different heating routine were further discussed in
relation to microstructural features.
1. Introduction
Dual phase (DP) steels are low-carbon and low-alloy steels with 10–30 vol.% of martensite and a ductile
ferrite matrix, which are widely used in the automotive industry because of the good combination of high
strength and good formability at low production cost [1, 2]. DP steels are characterized by a continuous
yielding behavior with a low initial flow stress and a high initial work hardening rate [3, 4]. Through
careful control of the chemistry and heat treatment path, dual-phase steel with different microstructure,
or different mechanical properties, can be obtained [5–7].
Recently, higher heating rate during continuous annealing treatment have received much attention
to produce ferrite-martensite DP steels. In the case of rapid heating during continuous annealing,
ferrite recrystallization is strongly suppressed during the heating stage, while the austenite formation
and its distribution are strongly influenced by overlapping of the processes of recrystallization and
austenitization and thus it could refine the grain structure, to improve both strength and ductility at
a
Corresponding author: dhshuang@mail.neu.edu.cn
This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits
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Article available at http://www.matec-conferences.org or http://dx.doi.org/10.1051/matecconf/20152108007
MATEC Web of Conferences
(a)
(b)
Figure 1. SEM micrographs showing microstructures in annealed dual-phase steel with varying heating rate. CA5 ◦ C/s (a), and HRA-300 ◦ C/s (b).
the same time [8]. The purpose of this study was to explore the influence of higher heating rate on
the microstructure and tensile behavior of DP590 steel. The strain hardening behavior and fracture
mechanisms were also studied in this presentation.
2. Experimental
An industrial Fe-0.08C-0.42Si-1.83Mn-0.16P-0.18Cr (wt.%) cold-rolled low carbon steel sheet with an
initial thickness of 1.5 mm was investigated in the present study.
In order to study the effect of higher heating rate on the microstructures and mechanical properties
of DP steel, the samples were annealed at inter-critical region (820 ◦ C/60s) with different heating rate
(5 ◦ C/s and 300 ◦ C/s) followed by water-quenching to form a DP microstructure.
To characterize the microstructure, the samples were grounded and then polished. After polishing,
the samples were etched with a 4 vol% nital solution for 10 s. The etched samples were used for
observing the microstructure by means of scanning electron microscope (SEM) on a FEI Quanta
600. The transmission electron microscopy (TEM) was carried out using FEI Tecnai G2F20S-TWIN
microscope. FEI Quanta 600 scanning electron microscope (SEM) with an OIM 4000 EBSD was used
to analysis the grain boundary characteristics of annealed samples.
After inter-critical annealing, tensile samples according to GB T228-2002 standard were cut from
the heated-treated steel. Tensile samples with a nominal gauge length of 50 mm and nominal width of
12.5 mm were used. The tensile test was carried out at across head speed of 3 mm/min using a 100 kN
Instron tensile machine. All specimens were tested to failure. During the tensile experiments, force and
displacement plots were recorded, from which stress and strain data was obtained.
3. Results and discussion
3.1 Microstructural evaluation
The microstructures of the heat-treated samples are shown in Fig. 1. It is evident that both treatments
resulted in ferrite-martensite DP microstructure, however, the morphology, size and distribution of
martensite phase varied significantly with the heat-treatment schedules. Compared with Fig. 1(a) with
a heating rate of 5 ◦ C/s, the sample with higher heating rate (300 ◦ C/s) have a fine grain structure and
martensite uniformly dispersed in the final DP microstructure. According to a quantitative measurement
by image-pro-plus, the amount of martensite in HRA sample was increased from 23% to 27%, while
the ferrite grain sizes was obviously refined to 5.3 m from 15.6 m as compared with the conventional
heating rate of 5 ◦ C/s in continuous annealing process.
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(a)
(c)
(b)
M
M
M
F
Figure 2. TEM micrographs of the annealed dual-phase steel heated with varying rate. Lath of martensite in
CA sample (a), twin of martensite in HRA sample (b) and lath of martensite in HRA sample (c). F: Ferrite; M:
Martensite.
(b)
(c)
80
Area percentage (%)
(a)
15°< ≤180°
2°< ≤15°
60
40
20
0
5
300
Heating rate (°C/s)
Figure 3. EBSD misorientation images of the annealed dual-phase steel heated with varying rate. CA-5 ◦ C/s
(a), HRA-300 ◦ C/s (b) (white grain boundaries 15◦ < ≤ 180◦ , black grain boundaries 2◦ < ≤ 5◦ ), and
misorientation between adjacent grains (c) measured from (a) and (b).
Figure 2(a) show the TEM images of CA sample, where the martensite shows coarse lath-like feature
and a few dislocations can be seen in the vicinity of martensite. However, in the case of HRA sample,
the martensite shows two typical substructure (lath and twin) and higher dislocation density in ferrite
matrix can be observed as marked by white arrow in Fig. 2(c).
Figure 3(a) and (b) show the EBSD orientation images and Fig. 3(c) shows the distribution
misorientation between adjacent grains measured from Fig. 3(a) and (b). The fraction of low angle
grain boundary was increased from 10.7% to 18.7% with the increase of heating rate. It is known that
low angle boundary is beneficial for the plastic and toughness of metal.
To understand the effect of higher heating rate on the microstructure of ferrite-martensite dualphase steels, it’s useful to consider the processing history of initial microstructure of steel. The received
steel had been cold-rolled and inter-critical annealing followed by water quenching. An important
consideration is austenite spatial distribution and content at the end of the inter-critical annealing cycle
will be inherited into the final microstructures after quenching.
In the case of CA sample, the ferrite recrystallization would fully complete when heated to the intercritical zone and then austenite nucleation at recrystallization ferrite grain boundaries and grows along
them [9, 10]. In addition, at lower heating rate, there are sufficient time for ferrite and austenite grains
to grow which produce larger ferrite and martensite grain.
In the case of HRA sample, the time from recrystallize start temperature (660 ◦ C, measured
by quantitative metallography) to inter-critical annealing temperature (820 ◦ C) is very short, only
0.53 s, which means that the ferrite have no time to recrystallize prior to reaching the inter-critical
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(a)
(b)
Figure 4. EPMA images of the annealed dual-phase steel heated with rates of CA-5 ◦ C/s (a) and HRA-300 ◦ C/s (b).
Engineering stress (MPa)
800
HRA
700
600
CA
500
400
300
200
100
0
0.00
0.05
0.10
0.15
0.20
0.25
Engineering strain (%)
Figure 5. Engineering stress-strain curves of dual-phase steel with varying heating rate.
temperature. In the unrecrystallized sample, carbide particles distributed along deformed ferrite grain
boundary which provides an increased nucleation density compared with the fully recrystallized samples
[11]. Subsequently, the austenite grows rapidly until they cover almost all the deformed ferrite grain
boundaries and at the same time the ferrite recrystallization occurs. The overlapping between ferrite
recrystallization and austenitization during annealing could refine the grain sizes. The amount of
austenite nuclei increases with the heating rate and rapid annealing could refine the grain sizes, the
more amount of austenite could also produce more martensite.
Figure 4(a) and (b) presents the distribution of carbon in annealed samples. Martensite is the mainly
rich carbon region, while, CA sample shows more uniform distribution than HRA sample. In the case
of HRA sample, higher carbon rich region can be observed as black arrowed in Fig. 4(b). This can be
attributed to the fact that a higher heating rate cause uneven distribution of carbon, in these regions twin
substructure of martensite could be produced.
3.2 Mechanical properties
Typical engineering stress-strain curves obtained from the tensile testing of the DP steel samples
subjected to different heating rate are presented in Fig. 5. Both curves show clear discontinuous yielding
characters of DP steel. Mechanical properties of different DP samples are summarized in Table 1. It can
be seen that HRA sample exhibit higher strengths and elongation. Compared with the CA sample, the
average yield strength Rp0.2 of HRA sample increases from 335 MPa to 351 MPa (4.7% higher), the
ultimate tensile strength Rm improves from 655.5 MPa to 706 MPa (4.7% higher), the total elongation is
enhanced from 22.4% to 25.0% (11.6% higher) and its uniform elongation from 13.8% to 15.4% (11.6%
higher).
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Table 1. Mechanical properties of the investigated steel after continuous annealing with varying heating rate.
HR (◦ C/s)
5
300
Rp0.2 (MPa)
335
351
Rm (MPa)
656
706.0
UEL (%)
13.8
15.4
TEL (%)
22.4
25.0
Rm× TEL (MPa %)
14683
17650
In general, the enhancement in strength is accompanied by a deterioration of elongation [4].
However, this does not apply to present study. This can be reflected in the strain hardening ability
and the fracture mechanisms of the steels.
3.2.1 Strain hardening behavior
A study in Ref. [12] showed that the modified C-J analysis method based on Swift equation can describe
the multistage strain hardening behavior, as expressed in Eq. (1):
= 0 + km .
(1)
Differentiating the above equation with respect to , we have the Eq. (2) in logarithmic form:
ln(d/d) = (1 − m) ln − ln(km)
(2)
where and are true stress and true strain, respectively, k is the constants which are normally called the
strength coefficient, 0 is the maximum elastic strain, (1−m) is the slope. It can be seen that the work
hardening ability decreases with the increase of the values of (1−m) since is an independent variable.
The plots of ln(d/d) versus ln for both the samples are shown in Fig. 6, also the values of (1−m) for
both stages in Fig. 6.
Analysis of strain-hardening ability of the generated DP microstructures by modified C-J technique
resulted into only two stages of strain-hardening irrespective of the heating rates as shown in Fig. 6.
Two stages of strain hardening have been demonstrated earlier for DP steels [13].
The lower slope in the first stage of both samples, indicating higher strain hardening is related to
the ferrite deformation restrained by the martensite. The second stage with higher slope, indicating
lower strain hardening ability, is due to the co-deformation of both ferrite and martensite [14]. However,
compared with CA sample, the HRA sample shows higher strain hardening ability in both stages.
This can be attributed to the fine grain structure and uniformly martensite distribution. Saeidi et al.
[15] reported, during the transformation from austenite to martensite by quenching, volume expansion
lead to geometrically necessary dislocation (GNDs) generation along the ferrite/martensite interfaces
in the ferrite grains. So the higher these interfaces in the fine grain sample, the higher density of
geometrically necessary dislocation present in the microstructure and the higher the strain hardening
ability. In addition, Balliger et al. [16] found that the strain hardening rate of DP steels increases
with a decrease in the martensite island size. Furthermore, uniformly martensite distribution lead to
reduction of strain partitioning between ferrite and martensite, thus, the strain hardening ability was
greatly improved.
A higher strain hardening rate delays the onset of necking and therefore, increases the uniform
elongation, this consistent with the measurement results in Table 1.
3.2.2 Fracture mechanism
The fracture surfaces of the investigated specimens are shown in Fig. 7. The fracture surfaces reveal
mixture of cleavage facets and dimples for both the specimens. However, the area of the cleavage
surfaces in the HRA sample is lower than the CA sample. It was observed that the size of dimples
in HRA sample is finer than CA sample. The reason for this phenomenon can be attributed to the
random and uniform disperse of martensite in ferrite matrix which restrict the growth of micro-voids.
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10.5
10.0
1-m =-2.8
ln (dσ/dε)
9.5
1-m =-3.2
−
9.0
8.5
1-m =-5.8
8.0
HRA
1-m =-6.0
7.5
7.0
CA
6.5
5.8
6.0
6.2
6.4
6.6
6.8
ln (σ)
Figure 6. Modified C-J plot of ln(d/d) versus ln() of annealed samples with varying heating rate.
(a)
(b)
Figure 7. SEM micrograph on the fracture surfaces of annealed samples with different heating rate (a) CA-5 ◦ C/s
and (b) HRA-300 ◦ C/s.
Some researchers [13, 17] have shown that large, banded and interconnected martensite islands in DP
steels, compared to the fine and isolated martensite particles, result in weaker fracture properties. They
observed that interconnected martensite distributed along ferrite grain boundaries cracked easily and
the fracture mode was predominantly cleavage type. While martensite cracking was less frequent and
the micro-voids were smaller and micro-voids density were higher in the specimen with fine martensite
particles and form the fracture was dimple depression type.
4. Conclusions
For DP590 steel, a good combination of strength and ductility can be achieved by rapid heating process,
and the work-hardening ability is also increased. Compared with the conventional heating rate (5 ◦ C/s)
in continuous annealing process, the ultimate tensile strength is improved from 656 MPa to 706 MPa
and the total elongation is enhanced from 22.4% to 25.0% after processed by rapid heating. In such a
case, the average ferrite grain in sample with higher heating rate was obviously refined to 5.3 m from
about 15.6 m, and the martensite morphology was changed into fine and uniformly dispersed in the
final microstructure from network along ferrite grain boundaries. Twinned substructure of martensite
occurred when heated at a higher heating rate in continuous annealing.
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